THE GEOLOGICAL HISTORY OF NEW YORK STATE SL 
‘being older than the Sierras and the Coast ranges. Much of the 
material making up the mass of the Catskill mountains was derived 
from the wear of the Taconic mountains, deposited in the sea just 
to their west and later raised high above sea level. 
SILURIC PERIOD 
The close of the Ordovicic age or the opening of the Siluric 
- found practically all the State above sea level and undergoing 
erosion. Along the eastern side the great Taconic range stood out 
prominently, but over the rest of the State we have no evidence 
that the land was very high. The central portion of the Adiron- 
dack region probably stood out somewhat more prominently than 
the western region. 
As shown on the geologic map (figure 1), the Siluric strata out- 
crops in a comparatively narrow belt which runs along the western 
side of the Hudson valley to the Helderberg hills, southwest of 
Albany, where it swings sharply around westward to follow the 
south side of the Mohawk valley, and thence as a somewhat wider 
belt along the south side of Lake Ontario. These Siluric strata 
everywhere dip under the Devonic (surface) rocks of the Catskill 
and Southwestern plateau provinces. This fact, combined with the 
knowledge that the strata are largely of widespread marine origin 
and also outcrop abundantly in central Pennsylvania, makes it 
practically certain that the Siluric rocks underlie all of the Catskill 
and southwestern plateau regions. Thus we must conclude that at 
least during much of Siluric time all of New York State south of 
Lake Ontario and the Mohawk vailey and west of the Hudson river, 
was covered by sea water. That the earliest Siluric sea did not 
spread over the area is proved by the absence of the very earliest 
known Siluric deposits. Furthermore, we know that the sea 
transgressed upon the State from the south or west, the Taconic 
range forming an effective barrier on the east and total absence of 
Siluric strata in the St Lawrence and Champlain valleys (as well 
as in Canada just north of the State) precluding encroachment of 
the sea from the north. 
This encroachment of the sea over so much of the State was due 
to a gradual sinking of the land. That central and western New 
York was submerged before the Hudson valley region is proved 
as follows: In central New York (south of Utica) the first deposit 
to form upon the eroded surface of the Ordovicic shales was the 
Oneida conglomerate which passes westward into the Medina sand- 
